1. Field of the Invention
This invention relates to improvements in a vibration damper for damping roll vibration of a power unit of an automotive vehicle under the effect of viscous and shearing resistance of a viscous fluid, and more particularly to a device for preventing the viscous fluid from leaking out of a casing even upon the vibration damper being subjected to a high temperature.
2. Description of the Prior Art
A variety of vibration dampers for damping roll vibration of a power unit under the effect of a viscous fluid have been proposed and put into practical use. A typical one of them is disclosed in Japanese Utility Model Provisional Publication No. 63-14036. This vibration damper includes a disc plate fixedly connected to a power unit. The disc plate is sealingly and movably disposed in a chamber defined in a casing connected to a vehicle body. A viscous fluid is filled in a space between the disc plate and the inner surface of the casing defining the chamber.
However, drawbacks have been encountered in such a vibration damper. That is to say, since the disc plate and the casing are formed of the same material having the same thermal expansion coefficient, the volume of the viscous fluid having a thermal expansion coefficient larger than that of the disc plate and the casing increases due to heat generated from an engine, and thereby rises the internal pressure of the casing. This degrades the fluid-tight seal of the casing, thereby causing leaking of the viscous fluid out of the casing.